Initial Experience with Wearable Cardioverter-Defibrillator
Abstract & Commentary
By John P. DiMarco, MD, PhD
Source: Chung MK, et al. Aggregate national experience with the wearable cardioverter-defibrillator event rates, compliance, and survival. J Am Coll Cardiol. 2010;56:194-203.
The wearable cardioverter-defibrillator (WCD) is a totally external system that can monitor a patient's cardiac rhythm and automatically treat episodes of ventricular tachycardia (VT) and ventricular fibrillation (VF) when they are detected. The WCD was market released in the United States in 2002, and patients who wore the WCD were followed in a database maintained by the manufacturer. This paper reports the experience from that database.
The WCD system includes electrodes for monitoring two surface electrocardiographic leads, nonadhesive defibrillation electrodes incorporated into a chest strap assembly, and a defibrillator unit (microprocessors, battery and capacitors) carried on a waist belt. If an arrhythmia is detected, an alarm sequence is automatically begun that includes vibrations, audible tones, and vocal commands. Patients can prevent shocks if they actively press buttons that will inhibit the shock. If the inactivation buttons are not pressed, the device is designed to charge, extrude gel from the defibrillation electrodes, and deliver up to five transcutaneous shocks at preprogrammed energy levels. Event electrograms are stored in the devices memory. No pacing can be delivered by the WCD, but bradycardic events are sensed and their electrograms stored.
The manufacturer's database contains indications, baseline patient demographics, rates of compliance (i.e., the proportion of time worn per day), and data from arrhythmic events. Electrograms for arrhythmic events were analyzed to determine if the shock was appropriate or inappropriate. Total mortality was assessed using the Social Security Death Index and compared to a group of patients from the Cleveland Clinic who received a first ICD implant between August 1996 and May 2004.
The manufacturer's database included information from 3,569 patients who wore the WCD for at least one day. The mean age was 59.3 years, and 74% of the subjects were male. The most common reasons for WCD use were patients who had an ICD explanted for infection (23%) and those with a history of ventricular tachycardia or fibrillation who were awaiting ICD implantation (16.1%). Current U.S. reimbursement guidelines restrict ICD implantation in patients with recent myocardial infarctions, recent coronary revascularization, and recently diagnosed nonischemic cardiomyopathy with depressed systolic function. These latter three groups accounted for 13%, 9%, and 20% of the entire cohort. The median duration of use was 36 days. There were only small numbers of patients who used the device for more than 120 days. Compliance was quite reasonable. Patients used the device a median of 21.7 hours per day. One in seven patients stopped wearing the WCD prematurely because of comfort issues or other adverse reactions.
During the time that the WCD was worn, the WCD recorded 80 sustained VT/VF events in 59 patients (1.7% of the total patient population). First-shock success was noted in 79 of 80 episodes. One patient had shock resistant, incessant VT, but this patient survived until pharmacologic therapy resulted in conversion. Eight patients died after successful first conversion of VT/VF, for an overall survival after a treated episode of 89.5%. Four of these patients died of recurrent arrhythmia. One patient's spouse prevented a second WCD discharge for a second episode ventricular arrhythmia. In two patients, recurrent arrhythmias were not shocked due to electrogram signal interruption caused, presumably, by the fall. One patient had interference with VF detection due to a unipolar pacemaker.
The WCD monitors for bradyarrhythmias but cannot treat them. There were 23 asystolic events, with 17 deaths and 3 deaths due to pulseless electric activity or respiratory arrest noted in patients while they were wearing the WCD, respectively. Inappropriate shocks were noted in 67 of the patients (1.9%). Inappropriate shocks resulted from inappropriate detections by the WCD and failure to suppress the shock using the response buttons by the patients.
Most of the patients who received appropriate shocks had traditional ICD indications. Thirty-three patients experienced 49 episodes of treated VT/VF. Six patients awaiting a scheduled ICD implant had nine events. Among 1,422 patients with nontraditional ICD indications, there were only 20 patients who had a total of 22 shocked VT/VF events. When survival among WCD recipients was compared to survival in a group of initial ICD recipients, the mortality curves were similar.
The authors conclude that prescription of a WCD is associated with acceptable rates of compliance and tolerance. VT/VF events are well detected and effectively treated. Patients awaiting ICD implant or reimplant with currently accepted indications are the most likely to benefit.
This report clearly shows that the WCD can provide effective therapy to patients at high risk for sudden death who are motivated enough to wear the device faithfully. The highest rate of use was in patients in whom a previously implanted ICD had been removed or in patients where other factors caused a scheduled ICD implant to be postponed. Use of the ICD for patients who did not have current indications for implant was infrequent and more data are needed before the WCD can be recommended for a wider group of patients. Two ongoing, randomized trials, the Vest Prevention of Sudden Death (VEST) trial and the Prediction of ICD Therapies study (PREDICT) are now underway and should provide these data for patients with recent myocardial infarction and depressed ejection fractions. I think it doubtful that the WCD will receive a high priority indication in any other large subgroups.